Process and apparatus for heating fluids in a well bore



March 19, 1968 F. w. BURTCH 3,373,8

PROCESS AND APPARATUS FOR HEATING FLUIDS 1N A WELL BORE Filed Jan. 6. 1.964

I W ITV INVENTOR. FRED V. l/RTC/ rrQRA/EV.

United States Patent C) 3,373,811 PROCESS AND APPARATUS FOR HEATING FLUIDS IN A WELL BORE Fred W. Burtch, Monroeville, Pa.,assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed Jan. 6, 1964, Ser. No. 335,852 20 Claims. (Cl. 166-25) This invention relates to a method for heating a Well bore penetrating a subterranean rock formation. In particular, this invention concerns a method for heating the production tubing in a well bore to prevent the deposition of paraflin from the produced fluids within the tubing and other well apparatus.

Certain oil-bearing underground rock formations contain crude oil having in solution significant amounts of waxy hydrocarbon substances generally known as parafflns. In this context, the term paraflin is defined as those normally liquid or solid organic compounds which are not soluble in the crude oil at all conditions of temperature and pressure normally existing in the producing well. As so defined, the parailins generally include the high molecular weight compounds of the various homologous series and consist of both straight and branch chained aliphatic hydrocarbons, aromatic hydrocarbons, naphthenes resins and asphalts. The solubility of the parafilns in the crude oil is sensitive to the temperature and the composition of the crude oil. Therefore, as the fluids produced from the formation are conducted upwardly through the tubing of the well to the surface, the reduction in temperature in and the evolution of volatile constituents from the crude oil cause the precipitation of paraflins from the oil and the accumulation of such precipitates on the walls of the tubing and other well apparatus. The temperature at which the paraflins begin to precipitate from solution in the crude oil is known as the cloud point.

Heretofore, many methods have been proposed for removing accumulated paraflln from the tubing, casing or other well apparatus. One such method employs various scraping devices which are reciprocated in the tubing or casing after the pumping rods and the pump plunger and valves have been removed from the well. This is a laborious and costly method for deparafllning well apparatus.

Various chemical methods have been suggested employing solvents which are injected into the well bore to dissolve the paraffin precipitates. Disadvantages in the use of solvents lare that generally not all of the paraflln precipitate is removed from the tubing and that treating the well tubing with a solvent requires shutting in production from the well during the treatment.

A commonly proposed method for deparaflining oil wells and for preventing the precipitation of paratins involves the application of heat to the well bore. One such method involves injecting heated fluids down the well through the annulus between the casing and the production tubing of the well. The additional cost of heating and injecting the fluids, and of the additional equipment required for the process, renders such a method unsuitable for general application to wells subject to the problem of paraffin accumulation.

Various types of heaters have been proposed for preventing paraffin precipitation in a well bore. Electrical heaters are unsatisfactory because they require the use.

of heavy armored electrical cables and are subject to frequent mechanical failures. The use of radioactive heating devices suspended in the tubing string has been suggested, but these devices are generally not satisfactory because of the danger of radioactive contamination of the produced fluids and the well apparatus and the necessity ice for closely controlling the reaction which generates the heat to assure that the reaction rate does not become excessive, thereby causing damage to the well apparatus. All of the various downhole heating devices heretofore suggested have an additional disadvantage in that they obstruct the flow of fluids through the tubing string and require larger diameter production tubing than would be necessary otherwise.

This invention resides in a method for heating a well bore penetrating an underground rock' formation to prevent the accumulation in the wellapparatus of paraffin precipitated from crude oil produced from the formations. The method comprises setting' throughout a predetermined interval of the well bore, a radioactive fission decay product that is chosen to assure that suflcient heat is liberated by the radioactive product to maintain the temperaure of the fluids in the well :bore near the casing or tubing wall above the cloud point of the produced oil.

FIGURE l is a longitudinal View, partially in cross section of a well apparatus prepared for an embodiment of the process of this invention in a well bore penetrating a subsurface rock formation.

According to the process of this invention, the temperature of the fluids in the tubing string, and of the tubing itself, must be maintained slightly abo-ve the cloud point of the produced oil. The cloud point of most paraffin-containing oils is less than 200 F. For example, samples of two crude oils from the Timbalier Bay Field, Louisiana, had gravities at 60 F. of 42.0 and 32.8 API and cloud points of and 150 F., respectively. An upper limit for the temperature of the fluids in the tubing is determined by the undesirability of heating the crude oil to a temperature which causes excessive vaporization of the lighter constituents in the oil. Both the cloud point and the maximum suitable temperature will differ for different oils and must be determined specifically for each individual application of the process of this invention. However, experience with the properties of paraffin-containing oils has indicated that the range of temperatures suitable for this process are readily attainable by the method of this invention.

The radiation accompanying the libration of thermal energy in the process of this invention may be any of the alpha, beta or gamma radiation normally emitted by the disintegration of radioactive fission decay products. Materials emitting alpha and beta radiation are more desirable because such radiation is considerably less penetrating than gamma radiation. Materials emitting beta radiation are preferred because that radiation is accompanied by the release of large quantities of heat. The radi-ation of neutrons liberated by the various fissionable materials is undesirable for the process of this invention because it requires the use of a moderating material to reduce the activity of the neutrons to thermal velocity and because the chain reaction must be closely controlled to prevent damage t-o the well apparatus.

Suitable radioactive materials should have a half-life adeq-uate to meet t-he heat requirements of the process during the anticipated producing life of the particular well. Therefore, a radioactive material having a half-life of at least 10 to 20 years is generally desirable, and a radioactive material having a half-life of 30 years or more is preferred.

Materials suitable as energy sources for the process of this invention can be chosen from among the various fission decay products produced by nuclear reactors and other devices employing fissionlable materials. Examples of the suitable radioactive materials found in fission decay products are strontium 9"-yttrium 90 and cesium 13'l-barium 13". These materials have a half-life of twenty years or more. A preferred material is strontium 9 because it has a half-life of approximately 30 years, is presently one o-f the most readily available fission decay products, and has a rate of heat release of approximately 3120 B.t.u./hr./lb. Other available fission `decay products having shorter halflives are cesium 1l-praseodymium 144 and promethium 147. It is not necessary that the radioactive material be introduced in its pure element-al form into the cement, but rather a crude precipitate of the radioactive substance can be used. For example, strontium sulfate (SrSO4) can provide heat at approximately 1370 B.t.u./hr./lb., and strontium titanlate (SrTiO3) can provide about 355 B.t.u./ hr./lb.

One preferred embodiment of the process of this invention comprises setting a sheath of a matrix material, containing the radioactive fission decay pro-duct, around the outside of a string of casing or tubing secured within the well bore. The matrix material suitable for use with that embodiment of the invention can be chosen from the solid highly viscous substances capable of sustaining a substantially uniform dispersion of the radioactive product throughout the matrix material at the temperature maintained by the evolution of heat from the radioactive product. For example, the fission decay product can be mixed with a gel or resinous substance which is set in the annulus around the casing. Drilling mud having thixotropic properties such that particles of the fission decay product will not settle out of the mud sheath is also a suitable matrix material. It is possible in certain instances to pack the annulus laround the casing or tubing with sand or gravel containing particles of a radioactive material.

The various oil well cements are a preferred matrix material because of their cost and availability and because they provide the additional benefit of supporting the casing or tubing in the well bore. Usually, the conventional oil well cements are satisfactory, but special high temperalture cements can be employed when unusually high ternperatures are required in la particular embodiment of this invention. The cements generally suitable for use with the process of this invention include the Pozzolanic cements and the hydraulic cements, of which Portland cement is a preferred type. Such cements are described in the publication entitled API Specification for Oil Well Cements and Cement Additives (API STD 10-A, sixth edition, January 1959) published by the American Petroleum Institute. If cement is used las a matrix material Ito secure the casing to the wall of the well bore, the mixture of cement with the radioactive material must have sufficient strength to support the casing. Ordinarily, a cement sheath having a strength of 100 to 200 p.s.i. is sufficiently strong but a strength of 500 p.s.i. or greater is preferred.

The concentration of radioactive material in the cement is partly determined by the required st-rength of the cement sheath. The upper limit of concentration of radioactive material in the cement must be less than the amount which lowers the strength of the cement below the minimum allowable strength for the particular Well. The lower limit of concentration of radioactive material in the cement sheath depends upon the specific radioactive material used and the minimum allowable rate of heat release to maintain the uids in the tubing above the cloud point -of the crude oil. Because the rate of release of energy by a fission `decay product declines continuously with increasing age of the product, the amount of thermal energy supplied by a fission decay product at the end of its half-life also must be considered in determining the minimum concentration of the radioactive product in the cement matrix. In t-he light of the requirement of a sufiiciently strong cement sheath that also supplies heat to the tubing at the required rate, a range of concentra-tion of radioactive material in the cement of from about 0.05 to about two percent by weight would be suitable for the presently available fission decay products. A more desirable range of concentration is from about 0.2 to about 1.5

percent by weight because it provides a high temperature in the tubing and a stronger cement sheath. To assure a uniformly strong cement sheath and the generation of an abundant quantity of Iheat for the complete prevention vof parafiin precipitation, the preferred range of concentration of radioactive material in the cement is from about 0.4 to about 1.2. percent by weight.

Generally, a satisfactory rate of heat release from the cement sheath is within a range of from about five to about 250 B.t.u./hr./ft. of cement in the well bore. A more desirable range of heat release is from 25 to 200 B.t.u./hr./ft. of cement because it does not cause excessive vaporization of the volatile constituents of the oil which would result in high producing gas-oil ratios and the possible precipitation of paraffins. To assure the complete prevention of paraffin deposition from the crude oil and to minimize the vaporization of the more volatile constituents of the oil, a preferred rate of heat release is within the range of from about 50 to about 150 B.t.u./ hr./ ft. of cement.

The emission of heat by radioactive fission decay products is essentially independent of pressure or of pressure variations within the range of pressures ordinarily encountered in a well bore penetrating a subsurface rock formation. However, a large rapid reduction in `pressure Within the flow string of a well can cause precipitation of parains owing to the cooling of the oil and the excessive liberation of volatile constituents from the oil. Such precipitation can be prevented by maintaining a back pressure on the fluids in the fiow string and by maintaining a sufiiciently high temperature in the oil flowing upwardly through the fiow string.

In the cementing procedure employed in placing a radioactive cement sheath in a well bore, the cement slurry is mixed at the surface and the radioactive material is added to the cement as the slurry is injected into the well bore. Small pellets of the fission decay product are encapsulated in a substance such as a ceramic material to facilitate handling at the surface.

Ordinarily, the temperature of the formation fluids will not fall below the cloud point until the produced fiuids have traveled some distance up the tubing. Therefore, a determination is made of the depth in the well bore above the producing formation at which the temperature of the fluids in the tubing falls below the cloud point ternperature. Then a string of surface casing is set in the well bore to a depth just below the depth at which the temperature in the well bore falls below the cloud point. A sheath of noneradioactive cement is then placed between the surface casing and the wall of the wel-l bore tO secure the surface casing in the well bore. Next, a string of production casing is run in the well bore from the Surface to the producing formation. A slug of radioactive cement, having a volume equal to that of the annulus between the surface casing and the production casing, is placed in the well bore. A slug of non-radioactive cement is displaced down the string of production casing to displace the radioactive cement upwardly in the annulus around the production casing so that the radioactive cement fills the annulus from the surface down to the bottom of the surface casing, The lower portion of the annulus between the wall of the well bore and the production casing, from the bottom of the surface casing to the bottom of the well, is then fu-ll of non-radioactive cement.

The production casing and the non-radioactive cement are perforated adjacent the producing formation and a string of production tubing is run into the well bore to the depth of the producing formation. A packer is set in the annulus around the tubing above the perforations and the annulus between the tubing and the production casing is filled above the packer with a thermally conductive liquid such as brine or drilling mud. Drilling mud is generally preferred because of its higher thermal conductivity.

The cementing procedure described above is presentedy merely by way of example and is not set forth as the exclusive method suitable for the process of this invention. In wells in which the leaching of radioactive materials into adjacent formations is not a problem, the surface casing can be omitted, and the radioactive cement can be used to secure the upper portion of the production casing directly to the wall of the well bore. In wells requiring the use of dual casing strings, the radioactive material can be suspended in a gel or other highly viscous liquid where a cement matrix is not needed to support the casing through the heated interval of the well bore.

A typical apparatus of a well in which the -process of this invention is employed is illustrated in FIGURE 1 wherein the well apparatus is indicated generally by the reference numeral 10. A well bore 12 penetrates a rock formation 14 which contains a crude oil having parafiins in solution. A string of surface casing 16 extends from a well head 13 to a depth 20 at which the temperature of fiuids in the well bore 12 falls below the cloud point of the crude oil. The surface casing 16 is secured to the wall of the well bore 12 by a sheath of conventional oil well cement 22. A string of production casing 24 is run into well bore 12 from the well head 18 to substantially the bottom 26 of we'll bore 12. The annulus between surface casing 16 and production casing 24 is filled, from the well head 18 to the depth 20 at the bottom of surface casing 16, by a sheath of cement 28 containing radioactive fission decay products. The annulus between production casing 24 and the wall of well bore 12 is filled by a sheath of conventional oil well cement 30 from the depth 20 at the bottom of surface casing 16 to the bottom 26 of well bore 12. A plurality of perforations 32 extend through the lower portion of production casing 24 and cement sheath 30 into formation 14. A string of production tubing 34 y extends from well head 18 downwardly through production casing 24 to substantially the bottom 26 of well bore 12, thereby forming an annulus 36 between tubing 34 and production casing 24. A packer 38 is set in annulus 36 around tubing 34 near the top of formation 14. The annulus 36 above packer 38 is filled with drilling mud to well head 18. The well apparatus depicted in FIGURE 1 is presented by way of example only, and variations in the arrangement of well apparatus are within the purview of this invention.

The procedure employed with the process of this invention can be explained in conjunction with its use in a well bore drilled through a subsurface rock formation at a depth of from 16,000 to 16,025 feet and containing a crude oil contaminated with parafiins. The well has a bottom hole pressure of 10,000 p.s.i.g. and a casing head pressure of 5,000 p.s.i.g. and produces 323 barrels of oil per day with a producing gas-oil ratio of 4430 s.c.f. of gas per :barrel of stock tank oil. The crude oil produced from the subsurface formation has a gravity of 31.5 API and a cloud point of 100 F. It is determined from other wells in the field that the formation temperature is 225 F. and that the temperature of the produced fluids flowing upwardly through the well bore does not decline to the cloud point until the fiuids reach a depth of 1700 feet below the surface. A string of surface casing having an outside diameter of inches is run into the well bore to a depth of 1700 feet, and a sheath of non-radioactive cement is set in the annulus between the surface casing and the wall of the lwell bore to secure the casing in the well. Next a string of production casing having an outside diameter of 7 inches is run in the well bore to a depth of 16,050 feet. The annular volume between the two strings of casing is equal to 0.364 cubic -feet per linear foot. The heat required to maintain the produced uids above the cloud point is equal to 95 B.t.u./hr. per linear foot of cement. This rate of heat output requires one unit of encapsulated fission production for each 164 units of cement if the fission product provides 95 B.t.u./hr./ft. of cement, and can be provided by using 1.54 pounds of strontium titanate per cubic foot of cement. The heat requirements of the well can also be satisfied by using a radioactive cement containing, 0.72 lb. of strontium9o per cubic foot of cement. Alternatively, an equivalent quantity of heat can be supplied using 0.92 lb. of strontium sulfate per cubic foot of cement.

In this embodiment of the process of this invention, the cement slurry for the radioactive cement is mixed using 530 sacks of type A Portland cement mixed with 2760 gallons of water. The radioactive material mixed with the cement slurry comprises 442 lbs. of strontium titanate to yield a concentration of radioactive material in the cement equal to 0.72 percent by weight. 'Ihe radioactive cement slurry is placed in the well bore and is displaced upwardly through the annulus around the 7 inch production casing by a volume of non-radioactive cement equal to the volume in the annulus around the 7 inch production casing extending from the bottom of the well bore to a depth 1700 feet below the surface. After the cement has set, the production casing and cement sheath are perforated over an interval extending from 16,000 to 16,025 feet, and a string of 2% inch production tubing is run in the well bore to a depth of 16,010 feet. A packer is set between the tubing and the production casing at a depth of- 15,950 feet. Prior to placing the well on production, the annulus formed by the 7 inch production casing and the 2% inch production tubing is filled with a drilling mud having a weight of from 12 to 14 llbs/ gal.

For illustrative purposes, the foregoing discussion of the process of this invention has dealt specifically with an embodiment whereby a radioactive fission decay product is set in a sheath of matrix material around a string of casing or tubing in the well bore. Another embodiment of this invention comprises suspending within the production tubing of the well an elongated receptacle containing a radioactive fission decay product which might or might not be mixed with a suitable matrix material. The receptacle herein employed can consist of a string of hollow sucker rods; a fiexible metallic tubing of stainles steel, copper or brass; an elongated sheet metal container; flexible synthetic tubing such as Tygon tubing; or any other means suitable for suspending a core of heat emitting radioactive material in the tubing while isolating the radioactive material from contact with fluids contained in the tubing.' Because of the small amount of radioactive material required to supply the quantity of heat necesary to the effective use of the process of this invention, a centralized heating core suspended in the tubing generally is of such small diameter that it does not obstruct substantially the ow conduit provided by conventionally used oil well production tubing. This embodiment has the advantage of being applicable in wells in which the necessity for well bore heating is not apparent until after the well is drilled and completed.

The preceding specification describes a method for maintaining the temperature in a well bore above the cloud point of crude oil produced from subterranean rock formations penetrated by the Well bore. The embodiment of this invention, whereby a sheath of radioactive material is set throughout the interval of a well lbore to be heated, has many advantages over the methods -proposed in the past for removing accumulations of paraffins from a wellbore or for preventing paraffin accumulation 'within the well bore. That method providesa single Well treatment which can be completed before the well is opened to production and which prevents the formation of paraffin accumulations in the well bore during the entire producing life of the well. In addition, the method of well lbore heating herein described is effective in maintaining highly viscous or paraffin crude oils in a sufficiently fluid condition to assure that they iiow readily throughout the entire ow string and do not become so viscous, as a result of cooling, that they plug the ow string above the producing formation. The use of a sheath of radioactive material around the casing of the well does not require the periodic shutting in of the well and does not obstruct the fiow conduits in the well.

Although this invention has been described with respect to a specific embodiment using radioactive cement as the well bore heat source, there are included Within the purview of this invention, any means by which heat is supplied to a well bore by inserting in the well bore a fission decay product mixed with a matrix material and set substantially permanently in the well bore as an integral part of the conventional well apparatus. Also included within the concept of this invention are those embodiments in which a radioactive fission decay product is incased in an elongated container which is suspended in the well bore within the flow string to heat the oil flowing therein. Generally, the heat requirements inherent in the process of this invention permit the use of such a centralized container having dimensions such that the fiow conduit provided by conventionally used oil field tubing is not substantially obstructed by the presence of the container. The embodiment employing a centralized core of radioactive material within the well tubing provides the obvious advantage of permitting a more direct heating of the oil with a consequent reduction in heat requirements.

Therefore I claim:

1. In a well bore penetrating a subsurface rock formation containing crude oil, a method for maintaining the temperature in the well bore above the cloud point of the crude oil comprising running in the well bore two concentric strings of casing to form an annulus in the well bore between said strings of casing, setting a sheath of matrix material in the annulus, said matrix material having substantially uniformly dispersed therethrough particles of a radioactive fission decay product capable of emitting heat at a rate adequate to maintain the temperature in the well bore above the cloud point of the oil.

2. A method according to claim 1 wherein the radioactive fission decay material is selected from a group consisting of strontiummyttrium90 and cesium137-barium137.

3. A method according to claim 1 wherein the matrix material contains the radioactive fission decay product in a concentration within a range from about 0.05 to about 2.0 percent by weight.

4. A method according to claim 1 wherein the radioactive fission decay material possesses a half-life with the range of about 10 to about 30 years.

5. A method according lto claim 1 wherein the radioactive fission decay material possesses a half-life substantially equal to the anticipated producing life of the well.

6. A method according to claim 1 wherein the radioactive fission decay material consists of strontium titanate in a concentration of at least 0.72 percent by weight and the matrix material is cement.

7. A method according to claim 1 wherein the radioactive fission decay material consists of strontium sulfate in a concentration o-f at least 0.92 percent by weight and the matrix material is cement.

8. A method according to claim 1 wherein the radioactive fission decay material is capable of providing heat to the well bore at a rate of at least 5 B.t.u./hr/ft.

9. In a well bore penetrating .a subterranean rock formation containing crude oil, said well bore having two concentric strings of casing forming an annulus therebetween, a method for maintaining the temperature in the well bore above the cloud point of the oil comprising setting in the annulus a sheath of cement mixed substantially uniformly with particles of a radioactive fission decay product, said radioactive product having a halflife of at least years and being capa-ble of releasing heat at a rate of at least five B.t.u./hr. per linear foot of said cement sheath, and said cement containing the radioactive product'in a concentration within a range of from about 0.05 to above 2.0 percent by Weight.

10. A method for preventing deposition of paraffins from crude oil in a well bore penetrating an oil-bearing subterranean rock formation comprising setting a string of surface casing in the well bore to a depth at which the oil produced from the formation cools sufficiently to permit the deposition of parafiins from the oil, running a strong of production casing through the surface casing from the surface to the formation, said production casing forming an annulus with the surface casing in the upper portion of the well bore, placing in the bottom of the well bore a slurry of radioactive cement of sufficient volume to ll the annulus between the surface casing and production casing, displacing a slurry of non-radioactive cement down the well bore and upwardly around the outside of the production casing and thereby displacing the radioactive cement upwardly around the outside of the production casing into the annulus between the surface casing and the production casing, said slurry of nonradioactive cement being of sufficient volume to substantially fill the well bore around the production casing from the bottom of the surface casing down to the bottom of the well bore, perforating the production casing and the non-radioactive cement adjacent the oil-bearing formation, running a string of tubing within the production casing to substantially the bottom of the well bore, setting a packer in the production casing around the tubing above the perforations, filling the production casing above the packer with drilling mud, and producing oil from the formation upwardly through the tubing wherein heat supplied to the well bore by the radioactive cement maintains the temperature of the oil above the temperature at which paraffin accumulation occurs.

11. A well for the production of oil containing paraffin comprising a bore hole extending downwardly to a formation containing said oil, a first casing extending from the surface part of the way downwardly to the formation containing said oil, a second casing extending downwardly within the first casing to a level below the lower end of the first casing, at least a portion of the annulus between the first casing and the second casing being filled with cement containing a radioactive fission decay product dispersed substantially uniformly therethrough, said fission decay product being of suicient concentration to maintain the temperature 0f the well bore above the cloud point of the oil.

12. A well for the production of oil containing paraffin from a subsurface formation containing said oil comprising a borehole extending downwardly from the surface, casing extending downwardly in said borehole at least a portion of the distance to said subsurface formation, conduit means extending downwardly through the casing to said formation for delivery of oil to the surface, and an elongated column of matrix material between said casing and conduit said matrix material containing a radioactvie fission decay product dispersed substantially uniformly therethrough, said fission decay product being of sufficient concentration to maintain the temperature of the well bore above the cloud point ofthe oil.

13. A method for heating oil in a well bore penetrating a subterranean rock formation comprising:

running a string of casing into the well bore; and filling an extended vertical length of the space between the casing and the formation with radioactive fission decay product capable of emitting heat at a rate adequate to maintain the temperature in the well bore above the cloud point of the oil. 14. A method for heating fiuids in a well bore penetrating a subterranean rock formation comprising:

running a string of casing into the well bore; and filling an extended vertical length of space between the casing and the formation with matrix material, said matrix material having disposed therethrough particles of radioactive fission decay product capable of emitting heat to fluids in said well bore at a rate of at least 5 B.t.u./hr./ft. of matrix material.

15. A method as set forth in claim 14 wherein the matrix material comprises cement. y

16. A method as set forth in claim 14 wherein th matrix material comprises a resinous substance which remains substantially solid during heating of the well bore.

17. A method as set forth in claim 14 wherein the matrix material comprises sand.

18. A method as set forth in claim 14 wherein the matrix material comprises gravel.

19. A method as set forth in claim 14 wherein the matrix material comprises drilling mud capable of sustaining the radioactive fission decay product in suspension during the heating of the Well.

20. A method as set forth in claim 14 wherein the 15 References Cited UNITED STATES PATENTS Odell 166-58 Buckley 166-5 Howell 166--5 Teplitz 166-5 Mihram et al. 1,66-421 Hanson 166-39 Netland 166-39 X Reistle 166-33 Robinson 166--140 Great Britain.

STEPHEN I NOVOSAD, Primary Examiner.

CHARLES E. OCONNELL, Examiner. 

1. IN A WELL BORE PENETRATING A SUBSURFACE ROCK FORMATION CONTAINING CRUDE OIL, A METHOD FOR MAINTAINING THE TEMPERATURE IN THE WELL BORE ABOVE THE CLOUD POINT OF THE CRUDE OIL COMPRISING RUNNING IN THE WELL BORE TWO CONCENTRIC STRINGS OF CASING TO FORM AN ANNULUS IN THE WELL BORE BETWEEN SAID STRINGS OF CASING, SETTING A SHEATH OF MATRIX MATERIAL IN THE ANNULUS, SAID MATRIX MATERIAL HAVING SUBSTANTIALLY UNIFORMLY DISPERSED THERETHROUGH PARTICLES OF A RADIOACTIVE FISSION DECAY PRODUCT CAPABLE OF EMITTING HEAT AT A RATE ADEQUATE TO MAINTAIN THE TEMPERATURE IN THE WELL BORE ABOVE THE CLOUD POINT OF THE OIL.
 11. A WELL FOR THE PRODUCTION OF OIL CONTAINING PARAFFIN COMPRISING A BORE HOLE EXTENDING DOWNWARDLY TO A FORMATION CONTAINING SAID OIL, A FIRST CASING EXTENDING FROM THE SURFACE PART OF THE WAY DOWNWARDLY TO THE FORMATION CONTAINING SAID OIL, A SECOND CASING EXTENDING DOWNWARDLY WITHIN THE FIRST CASING TO A LEVEL BELOW THE LOWER END OF THE FIRST CASING, AT LEAST A PORTION OF THE ANNULUS BETWEEN THE FIRST CASING AND THE SECOND CASING BEING FILLED WITH CEMENT CONTAINING A RADIOACTIVE FISSION DECAY PRODUCT DISPERSED SUBSTANTIALLY UNIFORMLY THERETHROUGH, SAID FISSION DECAY PRODUCT BEING OF SUFFICIENT CONCENTRATION TO MAINTAIN THE TEMPERATURE OF THE WELL BORE ABOVE THE CLOUD POINT OF THE OIL. 